The present invention relates to methods of preparing asbestos diaphragms useful in electrolytic cells utilized for the electrolysis of aqueous salt solutions, especially useful in electrolytic cells utilized for the electrolysis of aqueous alkali metal halide solutions, e.g., sodium chloride brine.
Diaphragms are used in electrochemical processes to separate an anolyte liquor from a catholyte liquor while permitting the flow of electrolyte there through. Diaphragms are used, for example, to separate an oxidizing electrolyte from a reducing electrolyte, a concentrated electrolyte from a dilute electrolyte, or a basic electrolyte from an acidic electrolyte.
In the electrolysis of an aqueous alkali metal halide solution, the diaphragm separates an acidic anolyte from an alkaline catholyte. Historically, commercial chlor-alkali diaphragms have been made of asbestos. Such diaphragms have been prepared by vacuum-drawing a liquid slurry containing asbestos fibers onto a porous cathode thereby depositing a mat of asbestos on the cathode. Asbestos diaphragms typically are characterized by a short lifetime of about 6 to 8 months.
Numerous efforts have been made to improve the lifetimes and performances of asbestos diaphragms. For example, according to U.S. Pat. No. 3,991,251, asbestos diaphragms can be strengthened by vacuum-depositing the asbestos from an aqueous slurry containing sodium hydroxide and heating the diaphragm to a temperature between about 110.degree. Centigrade (C) and 280.degree. C. for a sufficient period of time to react to the sodium hydroxide and the asbestos. However, such diaphragms can be too impermeable to the flow of electrolyte therethrough, thereby requiring a higher hydrostatic head of brine on the anolyte side of a diaphragm to maintain a desired electrolyte flow.
The following patents illustrate another technique of strengthening diaphragms by the use of materials, in particular fluorine-containing polymers, as binders with asbestos diaphragms. Generally, the technique includes mixing a slurry containing a particulate or fibrous binder material and asbestos fibers, depositing the solid materials as a mat on the porous cathode, and heating the diaphragm mat to sinter or melt the binder material, thereby effecting bonding between the asbestos and binder. For example, U.S. Pat. No. 4,065,534 describes the codeposition of a thermoplastic resin with asbestos followed by melting the resin to bind the asbestos. U.S. Pat. No. 4,070,257 describes the preparation of a diaphragm mat containing asbestos and a fluorine-containing resin binder, followed by sintering the resin by heating the diaphragm for about 1 to 10 minutes at a temperature in the range from the melting point to 100.degree. C. above the melting point of the resin. U.S. Pat. No. 4,142,951 describes depositing a diaphragm of crocidolite asbestos, chrysotile asbestos and a polymeric fluorocarbon resin followed by heating to sinter the polymer resin and provide bonding between the asbestos and resin. U.S. Pat. No. 4,410,411 describes the codeposition of a fluorine-containing polymer and asbestos followed by heating at a temperature sufficient to fuse, soften, and flow the polymer and thereby bind the diaphragm. Finally, Japanese Patent No. 55/073885 (1980) describes codeposition of a fluorine-containing polymer and asbestos from a slurry containing alkali metal hydroxide, followed by heating at a temperature from 200.degree. C. to 250.degree. C. to allow bonding of the asbestos and alkali metal hydroxide, but without melting the polymer.